(1) Field of the Invention
The present invention relates to easily-openable packaging containers and lids for the same. Particularly, the present invention relates to easily-openable packaging containers and lids for the same that have excellent sealing property since the lid members thereof can be heat-sealed so strongly as to obtain high pressure resistance to the pressure inside the containers, and yet have excellent easy openability. The easily-openable packaging containers of the present invention are suitable for packaging foodstuffs, especially for heat sterilization packaging.
(2) Description of the Prior Art
Flanged containers have been widely used for packaging foodstuffs or the like, and the flanged containers have been sealed by heat-sealing a lid member thereto after a content is filled. However, thus produced heat-seal containers have a defect that, if the sealing property is improved by increasing the heat-seal strength, the openability is aggravated resulting in the necessity for an opening operation using an edged tool or the like and, on the other hand, if a material having a proper interlaminar strength is selected to improve the openability, a uniform interlaminar strength cannot be obtained resulting in a decrease of the seal properties since the sealing property varies broadly depending upon the heat-seal conditions, such as, temperature, pressure, and time, or owing to foreign matters such as the content sandwiched between the heat-seal surfaces. In the latter case, the containers are inapplicable to packages in which the internal pressure increases at the time of heating sterilization such as boiling or retort treatment. That is, the conventional hermetically sealed containers, which is to be opened by peeling the lid member at their heatseal surfaces, could not reconcile the antipodal objects, secure sealing and easy openability.
For solving these problems, a method of providing a ring notch to the under-lid of a multilayer lid for cutting the under-lid at the time of peeling the lid for the facility of peeling is described in Japanese Utility Model Laid-open No. 148,742/1978. However, the method is applicable only to containers to be provided with a partial and hole-like opening, and it is inapplicable to containers to be opened at whole opening portion thereof, since, in the method, the sealing property is provided by spacing out the notch and the outer edge of the under-lid, and a part of the under-lid outside the notch remains protruding inside the opening after the opening of the container. That is, the containers produced by the above described method have a problem that they are inapplicable to drinks or desserts to be directly eaten or drunk since the contents cannot be discharged readily.